As a wonderful case study it seems generally agreed that the iTunes interface is horrific in Windows. Generally breaking conventions is bad. Making things look good is nice, but equating that with "just make it look like OSX" is going to cause you problems
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Ben Brocka♦Feb 27 '12 at 17:33

9 Answers
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You're introducing two big problems right off the bat; breaking conventions and clashing styles. Don't forget that Apple and Microsoft have released different interface guidelines for their respective platforms :
Windows UX Guidelines and the OS X UX Guidelines.

Using conventions is important and helps users work in your app without thinking (Don't Make Me Think!). Breaking conventions is even worse, because users will be confused and might initiate actions that don't apply to the current context. Context is important in how we interact with things; I see Windows 7 and I interact with my desktop a bit differently than I do in OSX--I might even interact differently than I do with Windows XP.

By breaking conventions people might not be sure what context applies; if I have a Mac App in Windows does it act like a Mac app or like Windows? You're immediately bringing up questions.

Remember you want to take inspiration, not copy design. People like Mac apps because they're simple, they work without errors and they're visually pleasing. "Copying" those attributes is great and works on any platform. Copying the specific styles that work in OSX like the brushed steel interfaces and borderless windows will simply introduce clash in a windows environment.

Let's take iTunes as an example:

The main window just looks out of place, and the brushed steel OSX look is oddly mixed with the Windows Vista glass theme. Note the dialog box has to use the default Windows Vista styles as well.

Clash isn't always bad, but if you app simply looks like you copied a Mac OS app, it's not going to be visually pleasing and it's going to introduce confusion as you're breaking those Windows conventions.

Perhaps most importantly making your apps look good shouldn't be a platform specific task. You shouldn't be thinking "How can I make a Windows app look like a nice Mac app?" you should be thinking "How can I make a Windows app look nice, like a Mac app?"

The users experience in your application should fit with the experience they have come to understand and work within in their chosen (or forced upon) OS. They have built laws and rules of interaction on that experience, for good or bad.

A major drawback with designing an app to look more like OS X UI than Windows 7 or Vista UI is that the overall look and feel of the app will not match the UI of Windows. Windows and OS X both have an experience which is analogous to each other; mouse driven interfacing, scrolling, double click, etc. The elements of design and interaction they do not share are jarring enough to bother many long time users of either system, once they experience it.

I think if you examine your application and decide to borrow certain concepts of information display from OS X you will find it more to your liking. For example, examine the differences between iTunes and Windows Media Player (Windows 7 version). A user has certain experiences within Windows they expect out of the dialog boxes, notifications, the task bar, etc.

But please note that Windows Media Player user interface (Windows 7 version) is confusing as hell, with multiple modes (each one with its own size and position), non-standard menus/widgets and floating mini-windows without basic controls and no docking option...
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marcusMay 5 '12 at 23:53

You are a mac user, so – naturally – you find your way around iTunes' interface, on whatever platform it may be running.

The users of the app, however, will most likely be windows users. They have learned the design principles and gui conventions windows offers, and will have an easy time learning a new app that sticks to these.

Which OS you perceive as more beautiful is subjective and not the point. Users expect certain interface aspects, even though they often cannot articulate them, they will be puzzled if they are missing.

There are also options that one OS offers and the other does not, which would require your user interface to "merge" the behavioral aspects of the os it runs on with the visual aspects of the os you try to mimic (the "maximize window" button coming to mind as a prime example).

Long story short: The success of your app will not be determined by it's "beauty", but by how easy and fast the users are able to efficiently use it.

The success of an app is typically tied to marketing. ;) That said, beauty--or rather, aesthetics certainly play into UX in that when used properly, do aid in efficient use of the app.
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DA01Feb 27 '12 at 18:02

The drawback is that it looks and behave like an OSX app on a windows machine. It would be equally bad to make an Android app that looks and behaves like an IOS app. Or a windows game that behaves like an console game. Each system has their conventions that you need to design around. It doesn't mean you need to stick to their established aesthetic, but it does mean you need to obey general layout rules such as putting the close button on the right side and not the left.

I suggest designing an app with a distinct visual style instead of relying on a OS to dictate the style. Take Chrome for example. They did not follow windows conventions at all, yet it is a very pleasant application to use. Mobile applications have been doing this for a while and the best ones are the apps that give a unique user experience to their users.

While I agree with you, it would be appropriate to explain why it's bad to ignore platform conventions, not just that it's bad. For instance, why do we "need to design around" system conventions? What is the impact of ignoring them?
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Kit GroseFeb 28 '12 at 3:56

There are two ways to view this question - one of of which is covered very well in the other answers and that is that you should avoid making a windows app look specifically like an OSX app.

But perhaps the question should be this - Is it ok to make an application on Windows look less like a windows app?

In which case you might lead on to ask: "Well what is meant by the look of a Windows app". To which one can only answer based on what the majority of users on Windows are used to. According to AT Internet and several others on Wikipedia, Windows 7 and Windows XP are in roughly equal use, each of which has about two and half times as much prominence as Vista. Thus it's not unreasonable to say that the following snapshot of XP is going to be pretty recognizable as Windows.

However, more and more software developers are wanting to target both Mac and Windows - and maybe Linux as well and hence cross platform toolkits like Qt (which also caters for some mobile platforms) are being used. By default Qt takes on the native look and feel automatically, but also allows customisation via CSS so that the same appearance can be used on all platforms while still using familiar OS dialogs like colour choosers, file open dialogs and the like.

This approach neither tries to make an application look like OSX on Windows nor like Windows on OSX, but allows the designer to create an interface that is a happy medium between the overly colourful more saturated hues of Windows and the more consistently designed reduced palette that OSX users are used to.

So you can quite well end up with nice looking applications like mFlow (below) which looks nothing like a traditional Windows app but has no problem fitting in. Similarly for Spotify which was also mentioned elsewhere who I believe developed their own cross platform GUI rendering toolkit. Skype is another application that has totally removed it's appearance from that of Windows to form it's own brand and identity.

I think the drawbacks of trying to do this then come from the potential to not do the job properly - or not considering the bigger picture. If you remove yourself from the notion of making a Windows app look like something else (OSX) and providing you don't just do half the job - making a Windows app look like a Windows app that's trying hard not to look like a Windows app (!), but instead you actually style the whole application to make it look better then there's no problem at all. That's so long as it befits the purpose of the application in the first place of course - that always takes priority.

Look at it like this :- If you're going to a top fashion event, don't just rely on a haircut to make you look good - dress well, choose the shoes carefully and clean your fingernails...!

This is where some technical understanding of the application is a must. How, exactly, are they porting the application over? Have they written their own UI layer? Or, more likely, are they leveraging existing UI layers that are platform-centric?

My guess is the latter, meaning that you can certainly design screen flows and layouts, but the core UI is going to be left to the OS, as it should be.

Now, are their exceptions? Yep. And, oddly, they tend to be the big players: Adobe and Apple.

Both have adopted a model of using their own UI layers so that the UI belongs to the software, not the OS.

Historically, it was a bad idea. Examples would be Mac software being ported to Windows while keeping the Apple 'chromeless' application metaphor. In OSX, an app doesn't have to have an open window to be running. In Windows it does. Another big difference was the menu bar. In OSX, it's always pinned to the top of the screen. In windows, it's always pinned to the top of the individual window.

So, go ahead and break conventions...but be SURE to understand the conventions first. You don't want to go breaking OS-centric conventions unless you have a really good reason to do so. I don't think assuming OSX is prettier is enough of a reason.

A major drawback of designing a Windows application to look like a Mac application is that it would look like a mac application.

I agree heaps with the comments that you should be trying to avoid clashing styles, and mainly sticking to the principle of least astonishment. I disagree with what feels like this collective feeling that mac interfaces are just so much better than windows interfaces, if you do go down that route make sure i can't right click on anything, and instead of getting error messages with horribly worded descriptions of what went wrong just popup a blank box and then hang.

I created and sold a Windows application years ago and had to answer the same question.

Drawbacks:

First, the user interface of a Mac app should look and function like modern popular apps on the OS X platform. This means that you will have to research your competitors on the Mac as well as download a variety of polished, popular apps on the App Store.

Remember that whether or not you rethink how the Mac app looks and works, you will still need to maintain, test, and answer questions about applications running on two different codebases.

On the positive side:

A dirty secret in indie game development ten years ago was that developers supporting dual platforms sold 50% of their games to Mac users even though only a few percent of users owned a Mac, because Mac users had more disposable income.

Now that we have the Mac App Store, laptops have taken over, and the most popular ones sold are Apple's, it is probably worth creating a Mac version even more than it has ever been.

Do you have a citation for the relative incomes being the dominant factor in the higher fraction of potential customers buying copies? I'd assume it was that it was a smaller market, and that in most cases only best selling PC games were ported. ex if the Windows gaming market had 20x as many games as there was time to play they would be bought on average by 1 gamer in 20. If the surplus for Mac games was only 2x, then the average mac game would be bought by half the audience.
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Dan NeelyFeb 27 '12 at 19:07